1. Field of the Invention
The present invention relates generally to the field of fluid-powered actuators, and, more particularly, to a piston-and-cylinder arrangement in which a radially-yieldable piston head may become axially tilted or cocked relative to an actuator rod in order to follow or track the bore of a cylinder.
2. Description of the Prior Art
Many forms of fluid-powered actuators have, of course, been developed heretofore. In a conventional piston-and-cylinder arrangement, a piston is mounted for sealed sliding movement along a cylinder bore. A first actuator rod may have a marginal end portion connected to the piston, and may have an intermediate portion extending away from the piston and penetrating an end wall of the cylinder. Often, a second rod is connected to the piston, and extends away from the piston in the opposite axial direction to penetrate the other end wall of the cylinder. The piston subdivides the cylinder into two opposing chambers, and has opposing faces arranged to face into these two chambers. The two piston faces may have the same cross-sectional area, or may have different areas. Pressurized fluid may be selectively admitted to, or withdrawn from, each chamber to exert a fluid force on the piston. Such force is a function of the area of the piston end face to which the fluid pressure is exposed, as well as the magnitude of that pressure.
In the case of an actuator having only one actuator rod connected to the piston, the areas of the opposing piston faces are normally unequal. The pressure in one chamber will act across the entire circular area of the piston. However, the pressure in the other chamber will act across the smaller-area opposite annular piston end face. These actuators are frequently formed such that the annular face of the piston has an area of about one-half of the opposite circular face. This means that, in equilibrium (i.e., in the absence of an external load), the pressure (P.sub.2) in the annular chamber will be about twice the pressure (P.sub.1) in the other chamber. Thus, even when the opposing fluidic forces acting on the piston are balanced (i.e., P.sub.1 A.sub.1 =P.sub.2 A.sub.2), a substantial pressure differential (i.e., P.sub.2 -P.sub.1) may exist across the piston.
In some applications, the cylinder is much elongated, and the actuator rod is designed to have a long stroke. The penetrant portion of the rod is typically supported by a bearing mounted on the cylinder end wall. As the rod is extended, the piston moves closer to this bearing, and the axial spacing between these two supports therefore decreases. If a load, whether externally applied or simply attributable to the weight of the extended rod, is not applied solely in an axial direction, such load will exert a force which urges the piston to tilt or cock relative to the cylinder. Such tilting or cocking increases the axial frictional force between the "relatively slidable" "piston and cylinder" surfaces, and causes unnecessary wear.
To prevent this from occurring, others have developed piston-and-cylinder arrangements having piston heads which "float" or move radially relative to an encircled portion of the actuator rod. For example, Sandau U.S. Pat. No. 3,785,253 discloses such a device in which a "floating" piston head is pressure-balanced, and therefore rendered insensitive to the magnitude of the fluid pressure (i.e., P.sub.1, P.sub.2) in either chamber and to any pressure differential (i.e., P.sub.2 -P.sub.1) across the piston head. However, Sandau pressure-balanced his "floating" piston head by providing four O-ring seals between various facing annular vertical surfaces which were arranged to move radially relative to one another. These face seals introduced substantial radial friction which impeded the ability of the piston head to "float" freely relative to the actuator rod. Other embodiments of prior art piston-and-cylinder arrangements are shown in U.S. Pat. Nos. 680,465 (Reynolds), 2,068,859 (Jones et al.), 2,460,948 (Sander), 3,207,426 (Gassman et al.), 3,247,767 (Aslan) and 3,556,538 (Muller).